BERLIN — What’s making toads puff up and explode in northern Europe? More than 1,000 toad corpses have been found at a pond in an upscale neighborhood in Hamburg and over the border in Denmark after bloating and bursting.

It’s left onlookers baffled. The pond water in Hamburg has been tested, but its quality is no better or worse than elsewhere in the city. The toad remains have been checked for a virus or bacterium, but none has been found.

One German scientist studying the splattered amphibian remains has a theory: Hungry crows are pecking out their livers.

“The crows are clever,” said Frank Mutschmann, a Berlin veterinarian who collected and tested specimens at the Hamburg pond. “They learn quickly from watching other crows how to get the livers.”

Based on the wounds, Mutschmann said, it appears that a bird pecks into the toad with its beak between the amphibian’s chest and abdominal cavity, and the toad puffs itself up as a natural defense mechanism.

But, because the liver is missing and there’s a hole in the toad’s body, the blood vessels and lungs burst and the other organs ooze out, he said.

“It’s not unique — it’s in a city area, and that makes it spectacular,” Mutschmann said. “Of course, it’s something very dramatic.”

There have also been reports of exploded toads in a pond near Laasby in central Jutland in Denmark.

Horror sceneLocal environmental workers in Hamburg have described it as a scene out of a horror or science fiction movie, with the bloated frogs agonizing and twitching for several minutes, inflating like balloons before they suddenly burst.

“The toads burst, the entrails slide out. But the animal isn’t immediately dead — they keep struggling for several minutes.”

Hamburg’s Institute for Hygiene and the Environment regularly tests water quality in the city and has found no evidence the toads were diseased. The institute also ruled out a fungus brought in from South America was infecting the toads.

Other theories have been that horses on a nearby track might have infected the amphibians with a virus, or even that the toads are committing suicide to save others from overpopulation.

Could hungry crows be a reasonable answer?

“We haven’t seen that. It might be, it might not be,” said institute spokeswoman Janne Kloepper. “It’s speculation,” until it’s observed, she added.

In the meantime, officials in Hamburg have advised residents to stay away from the pond, which German tabloids have dubbed “the death pool.”